Terrestrial pulmonate gastropods, slugs and snails (collectively, molluscs) are significant plant pests that affect commercial agriculture and horticulture and domestic gardens. These molluscs are omnivorous and consume large amounts of vegetative materials during their daily foraging. Consequently, they can seriously damage vegetable gardens and even plant crops during all phases of the growing cycle. Because of their destructive potential, control measures must be employed to ensure adequate protection of the growing plants from damage by terrestrial molluscs.
A wide variety of approaches have been used to try to combat pest molluscs. Perhaps the most common is the use of poisonous compounds called molluscicides. Molluscicides include a diversity of chemical compounds such as table salt (NaCl), calcium arsenate, copper sulfate, and metaldehyde. Molluscicides fall into two major groups, depending upon their mode of action: contact poisons and ingestible (or bait) poisons.
Contact poisons are molluscicides that, to be effective, must come into physical contact with the exterior of the mollusc, either by external application or through the action of the mollusc traversing a molluscicidal composition placed on the ground. The contact molluscicide is picked up by the proteinaceous slime coat of the mollusc and it builds up in the body of the mollusc until a lethal proportion is reached. One of the major drawbacks of contact molluscicides is that they have little effect if the molluscs do not physically contact the active chemical agent. If the molluscs are hidden or migrate into an area after a contact molluscicide is spread, the molluscs are unaffected. For these reasons, contact-acting mollusc poisons generally are considered to be unreliable.
Heavy metals, including zinc, aluminum, copper and iron, are all toxic to molluscs and are examples of compounds known to be effective molluscicides when used as contact poisons in the form of salts or chelates. See, Henderson, et al. Crop Protection (1990), 9, 131-134 and Henderson, et al., Ann. Appl. Biol. (1990), 116, 273-278.
Ingestible (or bait) mollusc poisons are those that must be ingested by a mollusc in order to be lethal. This type of mollusc poison tends to be preferred over contact poisons only because contact poisons, which rely upon passive acquisition of the active ingredient, are not considered to be reliable. One challenge associated with the development of effective bait molluscicides is to prepare a composition that is both palatable to the mollusc and effective as a lethal poison. Obviously, a sufficient quantity of the poison must be ingested to reach the lethal threshold. Often, compositions that are palatable to the mollusc are not effective as a lethal poison, while compositions that are quite potent and lethal are not readily ingested by molluscs. Many contact poisons, such as aluminum sulfate, copper sulfate and borax, are useless as ingestible poisons because they are not palatable to molluscs, and the molluscs do not ingest a lethal dose of these compounds. Ingestible poisons must be sufficiently palatable to the mollusc so that they will be consumed in lethal amounts, but the composition must also be slow acting enough to prevent the mollusc from becoming sick or cause it to cease feeding.
Typical problems associated with the development of compounds for the effective control of molluscs are discussed by Henderson, et al. in Aspects of Appl. Biol. (1986) 13, 341-347. This publication recognizes that although many compounds are known to be poisonous to molluscs, there is considerable difficulty in delivering the poison to the mollusc either as a bait or as a contact poison. The potential toxicity of a compound is irrelevant if molluscs will not consume a lethal dose of a bait poison.
Even if the molluscs will consume the bait, however, not all metal-containing compounds, e.g., metal chelates, will be effective. All metal chelates, even those in the same family, are uniquely different with respect to their biological effect on molluscs; not all such metal chelates are effective to kill molluscs even when the molluscs are injected with lethal doses of iron or similar metal. The effect of metal chelates on molluscs is a result of a unique physical reaction. The particular reasons why certain metal chelates are effective and others are not are unknown.
Accordingly, there remains a need for an effective ingestible poison for molluscs that is both palatable to molluscs and that does not pose a threat to the environment, crops, animals and other non-pests.